Two different calorimeters have been used to study various biological phenomena in body fluids. Measurements have been made of bilirubin binding to albumin and the heat output from washed erythrocytes has also been studied. Measurements of chymotrypsin, trypsin and pepsin activity in gastrointestinal secretions have been made. Differentiation of lactate dehydrogenase isoenzymes by calorimetry has been accomplished. Studies are continuing on the measurement of free and total cholesterol using specific enzymes.